In the production of electronic devices such as integrated circuits or liquid crystal displays, microlithography techniques have been used to form a patterned film of materials such as metal, which have desired electrical properties, on a certain substrate including a crystallized silicon wafer, a glass substrate, and the like. Microlithography comprises several steps including that of: forming a base layer of metallic materials on a substrate through chemical vapor deposition, plasma deposition or electroplating; applying a photoresist layer on the metal layer; exposing the photoresist layer under a photomask to light; developing the photoresist layer to provide a patterned photoresist layer; and etching the metal layer beneath the patterned photoresist layer by, for example, reactive etching to provide the metal wiring of a micro-pattern. Such multiple processes of microlithography, in addition to requiring the use of expensive photoresists and chemical etching material, make it undesirable in the light of cost as well as the protection of the environment. Moreover, many of the processes should be carried out under high temperatures and/or high pressures, and in such condition, diffusion of metallic vapor into the substrate is likely to occur, resulting in a deterioration of the final electronic devices. Recently, in the field of flexible display and TFT-LCD, demand has increased for requiring improved techniques of forming a high quality gate insulating film and a low resistance source/drain electrode area, and accordingly, diversified studies have been made to provide a more simplified method of forming a metal pattern.
For example, Japanese Laid-Open Publication No. 62-263973 discloses a patterning method, wherein an electron beam is irradiated on a thin film of organometallic compounds to form a metal pattern. In this method, an excessive amount of electron beam is necessary and thus mass-production of the metal pattern becomes difficult. Also, there are concerns about nonuniformity of the metal pattern.
In U.S. Pat. No. 5,064,685, an ink-containing organometallic compound is coated on a substrate, and, by exposure to a laser beam, the resulting coat is allowed to undergo thermal degradation to provide a patterned metal film. However, this method has a serious shortcoming in that the substrate should be subjected to high temperature conditions, and therefore a pattern of silver or silver alloy cannot be produced by this process.
On the other hand, U.S. Pat. Nos. 5,534,312 and 6,348,239 describe that a metal pattern can be obtained by coating a substrate with an organometallic complex synthesized by bonding one or more photosensitive organic ligands to one or more metal atoms, and exposing the coating to electromagnetic radiation, wherein any photosensitive resin needs not to be used. When exposed to electromagnetic radiation, the organometallic complex goes through a photochemical reaction, resulting in the dissociation of the organic ligands from the central metal atom. The remaining metal atoms then react with adjacent metal atoms and/or atmospheric oxygen atoms to form an oxidized metal pattern. However, there are several problems with ligand dissociation through photochemical reaction. Typically, the ligand dissociation rate is so low that a large amount of time is required to complete the patterning, and ligand contamination is inevitable. In addition, for improving the reflectivity of the oxidized metal pattern, heat treatment at 200° C. or more under a mixed gas stream of H2/N2 is required, and such treatment is not applicable to silver and silver alloys.
Thus, there remains a strong demand in the art to develop a method of producing a reflective film of silver or silver alloy under mild conditions while improving heat-resistance, adhesiveness and stability to discoloration caused by atmospheric oxygen.
At the same time, it should be realized that Silver is difficult to use in the production of a patterned film for numerous reasons: Silver is highly reactive with non-metallic elements, so it readily becomes discolored into a black or milky color, for example, by forming Ag2S or AgCl with sulfur or chloride in the atmosphere. In addition, silver is vulnerable to heat, so, in the case of producing a reflective film for LCD's by the use of silver, the process temperature should be controlled to prevent diffusion of the outer layer of the silver film. A reflective film comprising silver has a further problem in that yellow, reflected light is too strong at short wavelengths (i.e., 450 nm or less), and this yellowing problem gradually becomes severe. Therefore, silver has been seldom regarded as a useful material in the production of LCD's or PDA's.
In order to overcome these shortcomings of silver, Japanese Laid-Open Publication Nos. 01-221908 and 01-226765 disclose silver alloys containing 0.1˜3.0 wt % of Pt, Pd, or Rb and 0.1˜3.0 wt % of at least one of Cu, Ti, Cr, Ta, Ni, Mo, and Al. These alloys, however, are produced as a film on a substrate by sputtering so that microlithography processing is essential for obtaining a desired pattern, which is problematic in light of the complexity and expenses as described above.